Signal Conditioner - Easy and cheap?

[[email protected]]

I'll try to keep this short - if you need more info, please ask.

I have a sensor that has an output range of approximately -2mA to 2mA.
This output then goes into a computer. I'm trying to come up with a
simple/cheap/stable circuit which can take the sensor's mA output,
offset it by a constant, but adjustable amount and feed the new signal
to the computer.

Say today I see the sensor has an output of 1.3mA, but it needs to be
1.0mA, so I need the signal conditioner to offset by 0.3mA. Tomorrow,
I see the output is -0.7mA, but needs to be
-1.1mA, etc

I'm looking for some circuit that would be easily adjustable (variable
pot?) and relatively stable.

Any ideas?

Thanks in advance!
Corey

 

[jasen]

I'll try to keep this short - if you need more info, please ask.

I have a sensor that has an output range of approximately -2mA to 2mA.
This output then goes into a computer. I'm trying to come up with a
simple/cheap/stable circuit which can take the sensor's mA output,
offset it by a constant, but adjustable amount and feed the new signal
to the computer.

Say today I see the sensor has an output of 1.3mA, but it needs to be
1.0mA, so I need the signal conditioner to offset by 0.3mA. Tomorrow,
I see the output is -0.7mA, but needs to be
-1.1mA, etc

I'm looking for some circuit that would be easily adjustable (variable
pot?) and relatively stable.

Any ideas?

fix the software?

 

[[email protected]]

Thanks for the input, I wish it were that simple! Unfortunately, the
software is locked / static - no way to make a software correction.
I'll go into a little more detail.

This is a wideband O2 sensor. From my reading / testing, the sensor
has an output of approximately 0mA during stoichiometric combustion
(14.7:1 air fuel ratio), -2mA at the richest condition (11:1 AFR) and
about +1.5mA in pure air. It constantly switches from slight positive
current flow to slight negative current flow with a response time of
about 100 milliseconds, as the computer reads the sensor and tries to
keep the combustion at 14.7:1 AFR. For some various tests &
experiments I would still like the computer to control the combustion,
but do so at different (richer or leaner) conditions than the 14.7:1
which is 'hardwired" in.

My thought was perhaps a simple op-amp circuit which would be capable
of offsetting the O2 sensor output slightly to "fool" the computer.
For example, if the sensor is setting at 0mA and the circuit could
offset it to say -0.3mA the computer would think the combustion is
actually rich and pull some fuel out to balance the sensor back to 0mA.

Unfortunately, my mechanical skills are greater than my electronic
skills for the time being, so I was hoping someone on the forums might
be able to point out a relatively cheap / simple circuit that could do
the trick.

Regards,
Corey

 

[jasen]

Thanks for the input, I wish it were that simple! Unfortunately, the
software is locked / static - no way to make a software correction.
I'll go into a little more detail.

detail always helps.

This is a wideband O2 sensor. From my reading / testing, the sensor
has an output of approximately 0mA during stoichiometric combustion
(14.7:1 air fuel ratio), -2mA at the richest condition (11:1 AFR) and
about +1.5mA in pure air. It constantly switches from slight positive
current flow to slight negative current flow with a response time of
about 100 milliseconds, as the computer reads the sensor and tries to
keep the combustion at 14.7:1 AFR. For some various tests &
experiments I would still like the computer to control the combustion,
but do so at different (richer or leaner) conditions than the 14.7:1
which is 'hardwired" in.

sounds like ideally something that operates off the automobile's 12V supply
would suit, with battery powered a second option.

also voltage in the neighbourhood of 12V

My thought was perhaps a simple op-amp circuit which would be capable
of offsetting the O2 sensor output slightly to "fool" the computer.
For example, if the sensor is setting at 0mA and the circuit could
offset it to say -0.3mA the computer would think the combustion is
actually rich and pull some fuel out to balance the sensor back to 0mA.

yes sounds like an op-amp based current source could be a good solution.
what voltage is on the input to the ECU?

it's possible that going a little leaner will save fuel and increase fuel
economy, but it'll shut down the catalytic converter and you'll have
emissions problems.

Bye.
Jasen